1. Field of the Invention
This invention relates to a master information carrier carrying thereon an irregularity pattern representing the information to be transferred to a magnetic medium and a magnetic transfer method.
2. Description of the Related Art
With an increase in information quantity, there is a demand for a magnetic recording medium which is high in memory capacity, low in cost and preferably requires a short time to read out a necessary part of data (a magnetic recording medium which allows so-called high-speed access). As an example of such a magnetic recording medium, there has been known a high recording density magnetic medium such as a hard disc, a zip (Iomega) and the like. In such a high recording density magnetic medium, the recording area is formed by narrow data tracks. In order to cause a magnetic head to accurately trace such narrow data tracks and reproduce the data at a high S/N ratio, the so-called servo tracking technique has been employed.
In order to perform the servo tracking, it is necessary to write servo information such as servo tracking signals for positioning the data tracks, address signals for the data tracks and reproduction clock signals on the magnetic recording medium as a preformat upon production thereof. At the present, such preformat recording is performed by the use of a specialized servo recording apparatus (a servo track writer). However, the preformat recording by the conventional servo recording apparatus is disadvantageous in that it takes a long time since the servo information must be recorded on the magnetic recording medium one by one by the use of a magnetic head, which deteriorates the productivity.
As a method of recording the preformat accurately and efficiently, there has been proposed, for instance, in Japanese Unexamined Patent Publication Nos. 63(1988)-183623, 10(1998)-40544 and 10(1998)-269566, a magnetic transfer method in which a pattern which is formed on a master information carrier and represents servo information is copied to a magnetic recording medium (a slave medium) by magnetic transfer.
In the magnetic transfer, the magnetization pattern representing the information (e.g., servo information) carried by a master information carrier is magnetically transferred from the master information carrier to a slave medium by applying a transfer magnetic field to the slave medium and the master information in close contact with each other, and accordingly, the information carried by the master information carrier can be statically recorded on the slave medium with the relative position between the master information carrier and the slave medium kept constant. Thus, according to the magnetic transfer, the preformat recording can be performed accurately and the time required for the preformat recording is very short.
However, the master information carrier employed in the magnetic transfer disclosed in above identified Japanese Unexamined Patent Publications comprises a base sheet having on the surface thereof an irregularity pattern representing the information to be transferred and a magnetic layer formed at least on the surface of the protruding portions of the base sheet and since the magnetic transfer is carried out with the master information carrier and the slave medium held in close contact with each other, the magnetic layer can be partly lost or peeled off the pattern while the master information carrier is repeatedly brought into contact with and moved away from a number of slave media.
Removal of the magnetic layer from the master information carrier deteriorates contact between the master information carrier and the slave medium, which can result in deterioration in quality of the transferred signal. Further when the magnetic layer is peeled off the master information carrier over a wide area thereof, it is necessary to change the maser information carrier. The master information carrier is expensive and how many slave media can be made per one master information carrier largely governs the production cost of a magnetic recording medium.
We observed master information carriers from which the magnetic layer had been peeled and found that the magnetic layer was most peeled off the master information carrier at edges of flat top surfaces of the protruding portions of the irregularity pattern on the master information carrier and that as the pressure under which the master information carrier and the slave medium were in contact with each other increased, the magnetic layer was peeled off the master information carrier at a higher frequency. That is, it was presumed that dynamic structural weakness of the master information carrier caused the magnetic layer to be peeled off. The dynamic structural strength of the master information carrier is governed by, for instance, the thickness of the magnetic layer and the bonding strength between the base sheet and the magnetic layer. Our further investigation has revealed that edges of flat top surfaces of the protruding portions of the irregularity pattern on the master information carrier were often cut where the side faces of the protruding portions were covered with a very thin magnetic layer or with no magnetic layer, that is, the thickness distribution of the magnetic layer generated a structural strength distribution, and stress was concentrated on the portions where structural strength was weak to generate peeling of the magnetic layer.
Further, our investigation has revealed that the thickness of the magnetic layer on the side faces of the protruding portions of the irregularity pattern on the master information carrier largely affects the quality of the signal transferred to the slave media.